Channel access method for very high throughput (VHT) wireless local access network system

ABSTRACT

A channel access method for a very high throughput (VHT) system using a bonding channel having a plurality of subchannels is provided. The method includes receiving training information comprising a training offset value through a subchannel, performing channel estimation on a full channel bandwidth comprising all subchannels when a time corresponding to the training offset value is elapsed after the training information is received, transmitting a request to send (RTS) frame to a destination station through some subchannels selected from the plurality of subchannels by one or a plurality of source stations according to a result of the channel estimation, and transmitting a clear to send (CTS) frame to one source station selected from the plurality of source stations by the destination station in response to the received RTS frame. Accordingly, an effective channel access mechanism is provided for the VHT system, and collision among stations can be avoided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/201,844, filed on Sep. 27, 2011, now U.S. Pat. No. 8,948,102, whichis the National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2009/004414, filed on Aug. 7, 2009, which claimsthe benefit of U.S. Provisional Application No. 61/153,301, filed onFeb. 18, 2009, the contents of which are all hereby incorporated byreference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless local access network (WLAN),and more particularly, to a channel access mechanism for a very highthroughput (VHT) WLAN system and a station supporting the channel accessmechanism.

BACKGROUND ART

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local access network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted. In the initial WLAN technology, a frequency of 2.4 GHz wasused according to the IEEE 802.11 to support a data rate of 1 to 2 Mbpsby using frequency hopping, spread spectrum, infrared communication,etc. Recently, the WLAN technology can support a data rate of up to 54Mbps by using orthogonal frequency division multiplex (OFDM). Inaddition, the IEEE 802.11 is developing or commercializing standards ofvarious technologies such as quality of service (QoS) improvement,access point (AP) protocol compatibility, security enhancement, radioresource measurement, wireless access in vehicular environments, fastroaming, mesh networks, inter-working with external networks, wirelessnetwork management, etc.

In the IEEE 802.11, the IEEE 802.11b supports a data rate of up to 11Mbps by using a frequency band of 2.4 GHz. The IEEE 802.11acommercialized after the IEEE 802.11b uses a frequency band of 5 GHzinstead of the frequency band of 2.4 GHz and thus significantly reducesinfluence of interference in comparison with the very congestedfrequency band of 2.4 GHz. In addition, the IEEE 802.11a has improvedthe data rate to up to 54 Mbps by using the OFDM technology.Disadvantageously, however, the IEEE 802.11a has a shorter communicationdistance than the IEEE 802.11b. Similarly to the IEEE 802.11b, the IEEE802.11g implements the data rate of up to 54 Mbps by using the frequencyband of 2.4 GHz. Due to its backward compatibility, the IEEE 802.11g isdrawing attention, and is advantageous over the IEEE 802.11a in terms ofthe communication distance.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing speed of up to 540 Mbps at a frequency band of 5GHz, and is based on a multiple input and multiple output (MIMO)technique which uses multiple antennas in both a transmitter and areceiver to minimize a transmission error and to optimize a data rate.In addition, this standard may use a coding scheme which transmitsseveral redundant copies to increase data reliability and also may usethe OFDM to support a higher data rate.

Meanwhile, a basic access mechanism of IEEE 802.11 medium access control(MAC) is a carrier sense multiple access with collision avoidance(CSMA/CA) combined with binary exponential backoff. The CSMA/CAmechanism is also referred to as a distributed coordinate function (DCF)of the IEEE 802.11 MAC mechanism, and basically employs a “listen beforetalk” access mechanism. In this type of access mechanism, a station(STA) listens a wireless channel or medium before starting transmission.As a result of listening, if it is sensed that the medium is not in use,a listening STA starts its transmission. Otherwise, if it is sensed thatthe medium is in use, the STA does not start its transmission but entersa delay duration determined by the binary exponential backoff algorithm.

The CSMA/CA mechanism also includes virtual carrier sensing in additionto physical carrier sensing in which the STA directly listens themedium. The virtual carrier sensing is designed to compensate for alimitation in the physical carrier sensing such as a hidden nodeproblem. For the virtual carrier sending, the IEEE 802.11 MAC mechanismuses a network allocation vector (NAV). The NAV is a value transmittedby an STA, currently using the medium or having a right to use themedium, to anther STA to indicate a remaining time before the mediumreturns to an available state. Therefore, a value set to the NAVcorresponds to a duration reserved for the use of the medium by an STAtransmitting a corresponding frame.

One of procedures for setting the NAV is a exchange procedure of arequest to send (RTS) frame and a clear to send (CTS) frame. The RTSframe and the CTS frame include information capable of delayingtransmission of frames from receiving STAs by reporting upcoming frametransmission to the receiving STAs. The information may be included in aduration filed of the RTS frame and the CTS frame. After performing theexchange of the RTS frame and the CTS frame, a source STA transmits ato-be-transmitted frame to a destination STA.

FIG. 1 is a diagram showing an IEEE 802.11 MAC architecture including aDCF. Referring to FIG. 1, a service of the DCF is used to provide apoint coordination function (PCF) and a hybrid coordination function(HCF). The HCF includes enhanced distributed channel access (EDCA) andHCF controller channel access (HCCF). The HCF does not exist in an STAnot supporting quality of service (QoS). On the other hand, both the DCFand the HCF exist in an STA supporting QoS. The PCF is an arbitraryfunction in all STAs. Details of the DCF, PCF, EDCA, and HCCF aredisclosed in section 9 of the “MAC sublayer function description” in theIEEE 802.11-REVma/D9.0 October 2006 standard, and thus descriptionsthereof will be omitted herein. The contents of the above standard areincorporated herein by reference.

DISCLOSURE OF INVENTION Technical Problem

With the widespread use of a wireless local access network (WLAN) andthe diversification of applications using the WLAN, there is a recentdemand for a new WLAN system to support a higher throughput than a dataprocessing speed supported by the institute of electrical andelectronics engineers (IEEE) 802.11n. However, an IEEE 802.11n mediumaccess control (MAC)/physical layer (PHY) protocol is not effective toprovide a throughput of 1 Gbps or more. This is because the IEEE 802.11nMAC/PHY protocol is designed for an operation of a single station (STA),that is, an STA having one network interface card (NIC), and thus when aframe throughput is increased while maintaining the conventional IEEE802.11n MAC/PHY protocol, a resultant additional overhead is alsoincreased. Consequently, there is a limitation in increasing athroughput of a wireless communication network while maintaining theconventional IEEE 802.11n MAC/PHY protocol, that is, a single STAarchitecture.

Therefore, to achieve a data processing speed of 1 Gbps or more in thewireless communication system, a new system different from theconventional IEEE 802.11n MAC/PHY protocol (i.e., single STAarchitecture) is required. A very high throughput (VHT) system is a nextversion of the IEEE 802.11n WLAN system, and is one of IEEE 802.11 WLANsystems which have recently been proposed to support a data processingspeed of 1 Gbps or more in a MAC service access point (SAP). The VHTsystem is named arbitrarily. To provide a throughput of 1 Gbps or more,a feasibility test is currently being conducted for the VHT system using4×4 multiple input multiple output (MIMO) and a channel bandwidth of 80MHz.

Meanwhile, a data processing speed of 1 Gbps or more, which is set as atarget throughput in a VHT system, denotes an aggregate throughput. Onthe other hand, a target throughput in one-to-one communication betweenSTAs is at least 500 Mbps in the VHT system. This implies thatperformance or an offered load of an STA supporting VHT (hereinafter,simply referred to as a ‘VHT STA’) may not exceed 500 Mbps. In a casewhere the offered load of the VHT STA is less than 1 Gbps (e.g., 500Mbps), the target throughput of the VHT system cannot be achieved whenone VHT STA is allowed to use an entire channel similarly to theconventional channel access mechanism.

In addition, there is a problem in that efficiency is not high in theaforementioned carrier sense multiple access with collision avoidance(CSMA/CA) channel access mechanism used in the IEEE 802.11 WLAN. Forexample, a data processing speed in a MAC SAP is only 50 to 60 ? of adata processing speed in a PHY SAP. Therefore, in order to achieve adata processing speed of 1 Gbps or more in the MAC SAP of the VHTsystem, the data processing speed of the PHY SAP needs to be about 1.5to 2 times higher than 1 Gbps. However, the conventional IEEE 802.11nPHY technique has difficulty in providing such a processing speed.

In order to solve the aforementioned problems, the present inventionprovides a new channel access mechanism for achieving an aggregatethroughput of 1 Gbps or more in a VHT system.

The present invention also provides a channel access mechanism forallowing simultaneous channel access of a plurality of VHT stations(STAs) in a VHT system.

The present invention also provides a new channel access mechanism forachieving an aggregate throughput of 1 Gbps or more in a MAC SAP of aVHT system.

The present invention also provides a mechanism whereby STAs can useMIMO (or multi-user MIMO) in a full channel bandwidth.

The present invention also provides a mechanism whereby a channelestimation process can be performed without collision for a full channelbandwidth.

Technical Solution

According to an aspect of the present invention, a channel access methodfor a very high throughput (VHT) system using a bonding channel having aplurality of subchannels is provided. The method includes receivingtraining information comprising a training offset value through asubchannel; performing channel estimation on a full channel bandwidthcomprising all subchannels when a time corresponding to the trainingoffset value is elapsed after the training information is received;transmitting a request to send (RTS) frame to a destination stationthrough some subchannels selected from the plurality of subchannels byone or a plurality of source stations according to a result of thechannel estimation; and transmitting a clear to send (CTS) frame to onesource station selected from the plurality of source stations by thedestination station in response to the received RTS frame.

Advantageous Effects

According to an embodiment of the present invention, an effectivechannel access mechanism is provided to improve usage efficiency of abonding channel having a plurality of subchannels in a very highthroughput (VHT) system using the bonding channel. In particular,according to an embodiment of the present invention, multiple inputmultiple output (MIMO) or multi-used MIMO can be used by stations (STAs)in a full channel bandwidth, and a required channel estimation processfor the full channel bandwidth can be performed without collision amongthe STAs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an institute of electrical and electronicsengineers (IEEE) 802.11 medium access control (MAC) architectureincluding a distributed coordinate function (DCF).

FIG. 2 is a schematic view showing an exemplary structure of a wirelesslocal access network (WLAN) system according to an embodiment of thepresent invention.

FIG. 3 is a block diagram showing a multi-radio unification protocol(MUP) as an example of a protocol applicable to a very high throughput(VHT) system including a plurality of network interface cards (NICs)each having an independent radio interface.

FIG. 4 is a diagram showing an example of a channel access mechanism.

FIG. 5 is a diagram showing another example of a channel accessmechanism.

FIG. 6 shows a channel access method according to an embodiment of thepresent invention.

MODE FOR THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 2 is a schematic view showing an exemplary structure of a wirelesslocal access network (WLAN) system according to an embodiment of thepresent invention.

Referring to FIG. 2, the WLAN system includes one or more basis servicesets (BSSs). The BSS is a set of stations (STAs) which are successfullysynchronized to communicate with one another, and is not a conceptindicating a specific region. A very high throughput (VHT) BSS isdefined as a BSS that supports a super high-speed data processing of 1GHz or more.

A VHT system including one or more VHT BSSs can use a channel bandwidthof 80 MHz, which is for exemplary purposes only. For example, the VHTsystem may use a channel bandwidth of 60 MHz or 100 MHz or more. Assuch, the VHT system operates in a multi-channel environment where aplurality of subchannels having a specific size, e.g., a channelbandwidth of 20 MHz, are included.

The BSS can be classified into an infrastructure BSS and an independentBSS (IBSS). The infrastructure BSS is shown in FIG. 2. InfrastructureBSSs (i.e., BSS1 and BSS2) include one or more STAs (i.e., STA 1, STA 3,and STA 4), access points (APs) which are STAs providing a distributionservice, and a distribution system (DS) connecting a plurality of APs(i.e., AP 1 and AP 2). On the other hand, the IBSS does not include APs,and thus all STAs are mobile STAs. In addition, the IBSS constitutes aself-contained network since connection to the DS is not allowed.

The STA is an arbitrary functional medium including a medium accesscontrol (MAC) and wireless-medium physical layer (PHY) interfaceconforming to the institute of electrical and electronics engineers(IEEE) 802.11 standard, and includes both an AP and a non-AP STA in abroad sense. A VHT STA is defined as an STA that supports the superhigh-speed data processing of 1 GHz or more in the multi-channelenvironment to be described below.

The STA for wireless communication includes a processor and atransceiver, and also includes a user interface, a display means, etc.The processor is a functional unit devised to generate a frame to betransmitted through a wireless network or to process a frame receivedthrough the wireless network, and performs various functions to controlSTAs. The transceiver is functionally connected to the processor and isa functional unit devised to transmit and receive a frame for the STAsthrough the wireless network.

Among the STAs, non-AP STAs (i.e., STA 1, STA 3, STA 4, STA 6, STA 7,and STA 8) are portable terminals operated by users. A non-AP STA may besimply referred to as an STA. The non-AP STA may also be referred to asa wireless transmit/receive unit (WTRU), a user equipment (UE), a mobilestation (MS), a mobile terminal, a mobile subscriber unit, etc. A non-APVHT-STA is defined as a non-AP STA that supports the super high-speeddata processing of 1 GHz or more in the multi-channel environment to bedescribed below.

The AP (i.e., AP 1 and AP 2) is a functional entity for providingconnection to the DS through a wireless medium for an associated STA.Although communication between non-AP STAs in an infrastructure BSSincluding the AP is performed via the AP in principle, the non-AP STAscan perform direct communication when a direct link is set up. Inaddition to the terminology of an access point, the AP may also bereferred to as a centralized controller, a base station (BS), a node-B,a base transceiver system (BTS), a site controller, etc. A VHT AP isdefined as an AP that supports the super high-speed data processing of 1GHz or more in the multi-channel environment to be described below.

A plurality of infrastructure BSSs can be interconnected by the use ofthe DS. An extended service set (ESS) is a plurality of BSSs connectedby the use of the DS. STAs included in the ESS can communicate with oneanother. In the same ESS, a non-AP STA can move from one BSS to anotherBSS while performing seamless communication.

The DS is a mechanism whereby one AP communicates with another AP. Byusing the DS, an AP may transmit a frame for STAs associated with a BSSmanaged by the AP, or transmit a frame when any one of the STAs moves toanother BSS, or transmit a frame to an external network such as a wirednetwork. The DS is not necessarily a network, and has no limitation inits format as long as a specific distribution service specified in theIEEE 802.11 can be provided. For example, the DS may be a wirelessnetwork such as a mesh network, or may be a physical structure forinterconnecting APs.

FIG. 3 is a block diagram showing a multi-radio unification protocol(MUP) as an example of a protocol applicable to a VHT system including aplurality of network interface cards (NICs) each having an independentradio interface.

Referring to FIG. 3, an STA supporting the MUP includes a plurality ofNICs. The NICs are separately depicted in FIG. 3, which implies thateach NIC independently operates a MAC/PHY module. That is, the NICs aredistinctively depicted in FIG. 3 to show that the NICs are logicalentities operating according to individual MAC/PHY protocols. Therefore,the plurality of NICs can be implemented with physically distinctivefunctional entities, or can be implemented by integrating the physicalentities into one physical entity.

According to one aspect of the present embodiment, the plurality of NICscan be classified into a primary radio interface and one or moresecondary radio interfaces. If a plurality of secondary radio interfacesare present, the secondary radio interfaces can be classified into afirst secondary radio interface, a second secondary radio interface, athird secondary radio interface, etc. The classification into theprimary interface and the secondary interface and/or the classificationof the secondary ratio interface itself may be determined by a policy ormay be adoptively determined in consideration of a channel environment.

The plurality of NICs are integrally managed according to the MUP. As aresult, the plurality of NICs are externally recognized as if they areone device. For this, the VHT system includes a virtual-MAC (V-MAC).Through the V-MAC, an upper layer cannot recognize that a multi-radiochannel is operated by the plurality of NICs. As such, in the VHTsystem, the upper layer does not recognize the multi-radio channelthrough the V-MAC. This means that one virtual Ethernet address isprovided.

Next, a channel allocation mechanism in a VHT system will be describedaccording to embodiments of the present invention. Although theembodiments described below relate to a VHT system using a bondingchannel in which contiguous 4 subchannels having a bandwidth of 20 MHzare combined (i.e., a bonding channel having a channel bandwidth of 80MHz), this is for exemplary purposes only. The embodiments describedbelow can equally apply to a VHT system including a plurality ofsubchannels (e.g., 3 or 5 or more subchannels), which is apparent tothose skilled in the art. In addition, the embodiments of the presentinvention are not limited to the VHT system whose subchannel has abandwidth of 20 MHz.

FIG. 4 is a diagram showing an example of a channel access mechanism ina VHT WLAN system. In the present embodiment, the conventional channelaccess mechanism (e.g., enhanced distributed channel access (EDCA)mechanism) is directly used for an entire bonding channel, and it isassumed that a full channel bandwidth is used by only one VHT STA. Thatis, the entire bonding channel is used to exchange a request to send(RTS) frame, a clear to send (CTS) frame, and data between two VHT STAscommunicating with each other.

Referring to FIG. 4, a source VHT STA or a transmitting VHT STA, whichintends to transmit data, transmits an RTS frame by using the entirebonding channel. In FIG. 4, a process of transmitting the RTS frame isindicated by a physical layer convergence procedure (PLCP) preamble, aPLCP header, and a single PLCP protocol data unit (PPDU).

Upon receiving the RTS frame, a destination VHT STA or a receiving VHTSTA transmits a CTS frame also by using the entire bonding channel. InFIG. 4, a process of transmitting the CTS frame is also indicated by aPLCP preamble, a PLCP header, and a single PPDU.

When the RTS frame and the CTS frame are exchanged by using the entirebonding channel, subsequent data or the like is generally transmittedalso by using the entire bonding channel. However, according to oneaspect of the present embodiment, the RTS frame and/or the CTS frame mayinclude a list of subchannels to be used for transmission of subsequentdata or the like. As such, when the RTS frame and/or the CTS frameinclude the list of subchannels, a network allocation vector (NAV) isset only for a specific subchannel included in the list, and the sourceVHT STA transmits the data or the like to the destination VHT STA onlythrough the specific subchannel.

Consequently, upon receiving the CTS frame, the source VHT STA startstransmission of the data or the like to the destination VHT STAaccording to a predetermined procedure. In FIG. 4, a process oftransmitting the data is also indicated by a PLCP preamble, a PLCPheader, and a single PPDU. In this case, if the list of subchannels doesnot exist in the RTS frame and/or the CTS frame, as shown in FIG. 4, thedata or the like is transmitted by using the entire bonding channel.Otherwise, if the list of subchannels exists, the data or the like maybe transmitted by using all or some of subchannels included in the list.

As described above, according to the embodiment of the presentinvention, the RTS frame, the CTS frame, the data frame, etc., aretransmitted through the entire bonding channel directly using thechannel access mechanism based on the conventional EDCA. As amodification of the embodiment, the list of subchannels to be used fortransmission of the data frame or the like may be included in the RTSframe and/or the CTS frame. If the list of subchannels is included, thesource VHT STA can transmit the data frame or the like to thedestination VHT STA by using all or some of subchannels in the list.

In a case where the RTS frame and the CTS frame are transmitted by usingthe entire bonding channel, the RTS frame and the CTS frame are verysmall in size, and thus a transmission time corresponds to only a feworthogonal frequency division multiplex (OFDM) symbols (e.g., 8 ?srequired for transmission of 6 Mbs). Optionally, the transmission timeof the RTS frame and the CTS frame may be less than the PLCP preambleand the PLCP header. A network overhead for the RTS frame and the CTSframe is almost negligible.

According to the present embodiment, when collision occurs with thelegacy STA, the entire bonding channel cannot be used, which may resultin significant deterioration in a throughput of the VHT system. If oneor more legacy STAs simultaneously operate in any one subchannel among aplurality of subchannels used or to be used by a VHT STA, allsubchannels constituting the channel or the bonding channel must be inan idle state in order for the VHT STAT to access to a channel includingthe subchannel or the entire bonding channel. That is, the VHT STA cansuccessfully perform channel access when there is no collision with thelegacy STA with respect to all subchannels constituting the bondingchannel.

In addition, according to the present embodiment, when the legacy STAoccupies any one subchannel at a time of transmitting the RTS frame,even if the use of the subchannel is finished some time later, the VHTSTA cannot transmit data or the like by directly using the entirebonding channel. That is, only after the use of the subchannel by thelegacy STA is finished, an exchange process of the RTS frame and the CTSframe can start.

FIG. 5 is a diagram showing another example of a channel accessmechanism in a VHT WLAN system. The present embodiment is an example ofa channel access mechanism for preventing collision among VHT STAs in aVHT system in which a VHT STA coexists with a legacy STA or in a VHT BSSin which only a VHT STA exists. Such a channel access mechanism can alsobe referred to as, for example, frequency-hopping EDCA with dynamicchannel allocation.

According to a channel access mechanism using a frequency-hopping EDCAscheme, a destination VHT STA can receive RTS frames simultaneously froma plurality of UEs, or can receive an additional RTS frame through asubchannel currently not used. In this case, according to the presentembodiment, CRT frames are respectively transmitted to one or more UEswhich have transmitted the RTS frames, and thus several UEs cansimultaneously transmit data or the like through different subchannels.The CTS frame includes a list of subchannels to be used when acorresponding UE transmits data or the like.

Referring to FIG. 5, source VHT STAs or transmitting VHT STAs (indicatedby ‘STA 1’ and ‘STA 2’ in FIG. 5), which intend to transmit data,transmit RTS frames through respective subchannels. For example, thismay be a case where backoff timers of the STA 1 and the STA 2 aresimultaneously expired.

When the backoff timers are expired, the VHT STAs transmit RTS frames.Only one subchannel is used in the transmission of the RTS frames. Thesubchannel can be selected in various manners. For example, thesubchannel can be randomly selected. The VHT STAs attempt channel accessby using the EDCA scheme.

For example, four subchannels of 20 MHz exist in a VHT system having achannel bandwidth of 80 MHz. The VHT STA transmits an RTS frame byrandomly selecting one subchannel from the four subchannels.

As such, according to the present embodiment, RTS frames are transmittedby using only one subchannel, and thus even if a plurality of VHT STAssimultaneously transmit the RTS frames, collision among the RTS framescan be prevented or avoided. That is, as shown in FIG. 5, the backofftimers of the STA 1 and the STA 2 may be simultaneously expired, andsubsequently the STA 1 and the STA 2 respectively transmit RTS frames.However, since the STA 1 and the STA 2 transmit the RTS frames throughdifferent subchannels, collision does not occur when the RTS frames aretransmitted by the respective STAs.

It is shown in FIG. 5 that the STA 1 uses a 1st subchannel and the STA 2uses a 3rd subchannel when the RTS frames are transmitted, which is forexemplary purposes only. According to the present embodiment,preferably, the STA 1 and the STA 2 transmit the RTS frames by usingdifferent subchannels, and the subchannels can be determined without anyrestriction.

That is, when the STA 1 and the STA 2 transmit the RTS frames by usingdifferent subchannels, collision among the RTS frames can be prevented.In FIG. 5, a process of transmitting the respective RTS frames by theSTA 1 and the STA 2 through the different subchannels is indicated by aPLCP preamble, a PLCP header, and a single PPDU in the 1st and 3rdsubchannels.

When the destination VHT STA or the receiving VHT STA receives separateRTS frames through the 1st and 3rd subchannels among all subchannels,the destination VHT STA or the receiving VHT STA transmits CTS frames inresponse to the all received RTS frames.

A CTS frame is transmitted through an entire bonding channel when theCTS frame is transmitted. By transmitting the CTS frame in such amanner, a channel load caused by transmission of the CTS frame can bereduce. However, if the CTS frame is transmitted through the entirebonding channel, the legacy STA cannot decode the CTS frame, and as aresult, cannot set the NAV during a time period determined by the CTSframe. Accordingly, in the present embodiment, the CTS frame istransmitted by using one subchannel.

The CTS frame may be transmitted for each subchannel constituting thebonding channel.

In FIG. 5, in response to RTS frames received from a 1st VHT STA (i.e.,STA 1) and a 2nd VHT STA (i.e., STA 2), two CTS frames are respectivelytransmitted to the STA 1 and the STA 2 through 1st and 3rd subchannels,which is for exemplary purposes only. That is, an AP receives two RTSframes in total from the STA 1 and the STA 2, and thereafter transmitsCTS frames to the STA 1 and the STA 2 in response to the received RTSframes.

In FIG. 5, a process of transmitting the CTS frames is indicated by aPLCP preamble, a PLCP header, and a single PPDU in each of the 1st and3rd subchannels.

In this case, the CTS frame may include a subchannel list to indicatefor which subchannel a transmission opportunity is provided for each VHTSTA. For example, if transmission is allowed for one VHT STA, asubchannel list that can be used by the VHT STA may be included in theCTS frame. If no subchannel list is included, the VHT STA may have atransmission opportunity for all subchannels.

According to the present embodiment, a list of subchannels to be used bythe STA 1 to transmit subsequent data or the like is included in a CTSframe to be transmitted to the STA 1. According to the presentinvention, 1st and 2nd subchannels are included in the list, which isfor exemplary purpose only. Likewise, a list of subchannels to be usedby the STA 2 to transmit subsequent data or the like is also included ina CTS frame to be transmitted to the STA 2. According to the presentembodiment, 3rd and 4th subchannels are included in the list, which isfor exemplary purpose only.

Upon receiving the CTS frame, each of the STA 1 and the STA 2 transmitsdata or the like to the destination STA through a subchannel included inthe subchannel list of the received CTS frame. The STA 1 and the STA 2can simultaneously transmit the data or the like. In FIG. 5, a processof transmitting the data through 1st and 2nd subchannels of the 1st VHTSTA and transmitting the data through 1st and 2nd subchannels of the 2ndVHT STA is indicated by a PLCP preamble, a PLCP header, and a singlePPDU in the 1st and 2nd subchannels and the 3rd and 4th subchannels.

According to the embodiment of the present invention, a plurality of VHTSTAs or a VHT STA and a legacy STA can transmit data or the like byusing an entire bonding channel. In addition, according to theembodiment of the present invention in which a CTS frame includes a listof subchannels to be used, a VHT STA for using each subchannel can beadaptively determined by considering all conditions at the request ofthe plurality of VHT STAs. Therefore, according to the presentembodiment, channel usage efficiency can be improved when transmittingdata or the like.

FIG. 6 shows a channel access method according to an embodiment of thepresent invention. In comparison with the example described withreference to FIG. 5, the channel access method of FIG. 6 furtherincludes a channel estimation process. In FIG. 6, a PLCP frame formatfor the frequency-hopping EDCA scheme is proposed.

In order for STAs to use MIMO (or multi-user MIMO) in a full channelbandwidth (e.g., 80 MHz), the channel estimation process is required forthe full channel bandwidth corresponding to a bonding channel. Thechannel access method proposed in the embodiment of the presentinvention includes the channel estimation process. Therefore, the STAscan use MIMO by using the channel access method including the channelestimation process according to the embodiment of the present invention.

According to the PLCP frame format proposed in the present embodiment, ashort training field and a long training field for the full channelbandwidth corresponding to the bonding channel are transmitted after aspecific training offset. The short training field and the long trainingfield are transmitted through the bonding channel.

The PLCP frame format and the channel estimation process using the PLCPframe format will be described with reference to FIG. 6. Herein, channelaccess is performed by STAs conforming to the aforementioned frequencyhopping EDCA scheme.

A legacy-short training field (L-STF), a legacy-long training field(L-LTF), and a legacy-signal (L-SIG) are respectively a short trainingfield, a long training field, and a signal for a legacy STA. Inaddition, a VHT-short training field (VHT-STF), a VHTLong Training Field(VHT-LTF), and a VHT-Signal (VHT-SIG) are respectively a short trainingfield, a long training field, and a signal for a VHT STA.

If it is assumed that a bonding channel consists of four subchannels(i.e., 1st to 4th subchannels), an STA 1 and an STA 2 respectively use a1st subchannel 610 and a 3rd subchannel 615 for example. The 1stsubchannel 610 is used between an AP and the STA 1. The 3rd subchannel615 is used between the AP and the STA 2. L-STF, L-LTF, and L-SIG sets620 and 625 are transmitted respectively through the 1st subchannel 610and the 3rd subchannel 615. Since the L-STF, L-LTF, and L-SIG set 620and the L-STF, L-LTF, and L-SIG bundle set 625 are transmitted throughdifferent subchannels, they can be simultaneously transmitted.Subsequent to the transmission of the L-STF, L-LTF, and L-SIG sets 620and 625, a VHT-SIG 630 and a VHT-SIG 635 are transmitted respectively tothe STA 1 and the STA 2 through the 1st subchannel 610 and the 3rdsubchannel 615.

The VHT-SIGs 630 and 635 include a training bandwidth value and atraining offset value.

The training bandwidth value denotes a channel bandwidth to be used fortransmission of VHT-STFs 640 and 645 and VHT-LTFs 650 and 655, that is,a channel bandwidth to be used for channel estimation. In the embodimentdescribed with reference to FIG. 6, the STA 1 and the STA 2 transmit theVHT-STFs and VHT-LTFs 640, 645, 650, and 655 by using an entire bondingchannel instead of any one subchannel. Therefore, in this case, atraining bandwidth value included in the VHT-SIGs 630 and 635 is 80 MHz.This is common to the VHT-SIG 630 transmitted by the STA 1 and theVHT-SIG 635 transmitted by the STA 2.

The training offset value denotes a time difference between a time whenthe VHT-SIGs 630 and 635 are transmitted and a time when the VHT-STFs640 and 645 and the VHT-LTFs 650 and 655 are transmitted. The trainingoffset value may indicate positions of the VHT-STFs 640 and 645 and theVHT-LTFs 650 and 655 in a PLCP frame depicted along a time axis in FIG.6.

For example, if the VHT-SIGs 630 and 635 have a training offset value of0, the VHT-STFs 640 and 645 and the VHT-LTFs 650 and 655 are transmittedimmediately after the VHT-SIGs 630 and 635 are transmitted.

If the VHT-SIGs 630 and 635 have a training offset value of 1, theVHT-STFs 640 and 645 and the VHT-LTFs 650 and 655 are transmitted when atime for transmitting one set of VHT-SIG, VHT-STF, and VHT-LTF iselapsed after the VHT-SIGs 630 and 635 are transmitted. This impliesthat another UE may transmit the VHT-STF and VHT-LTF in advance.

If the VHT-SIGs 630 and 635 have a training offset value of 2, theVHT-STFs 640 and 645 and the VHT-LTFs 650 and 655 are transmitted when atime for transmitting two sets of VHT-SIG, VHT-STF, and VHT-LTF iselapsed after VHT-SIG transmission. That is, a time for transmitting theVHT-SIG, VHT-STF, VHT-LTF, VHT-SIG, VHT-STF, and VHT-LTF corresponds tothe training offset value of 2. This implies that two other UEs maytransmit the VHT-STF and VHT-LTF in advance.

If the VHT-SIGs 630 and 635 have a training offset of 3, the VHT-STFs640 and 645 and the VHT-LTFs 650 and 655 are transmitted when a time fortransmitting three sets of VHT-SIG, VHT-STF, and VHT-LTF is elapsedafter VHT-SIG transmission. That is, a time for transmitting theVHT-SIG, VHT-STF, VHT-LTF, VHT-SIG, VHT-STF, VHT-LTF, VHT-SIG, VHT-STF,and VHT-LTF corresponds to the training offset of 3. This implies thatthree other UEs may transmit VHT-STF and the VHT-LTF in advance.

As such, the training offset is differently set for each STA or for eachsubchannel, and thus it is possible to avoid collision among STAs thatsimultaneously access to a channel to perform the channel estimationprocess.

The training offset may be determined according to a subchannel selectedby a STA or may be randomly determined by the STA.

By using the training offset, STAs simultaneously accessing a channel inthe frequency-hopping EDCA process can perform the channel estimationprocess without collision for an entire bonding channel, i.e., a fullchannel bandwidth.

That is, after the STA 1 and the STA 2 transmit the VHT-SIGs 630 and 635through the respective subchannels 610 and 615, the STA 1 firsttransmits the VHT-STF 640 and the VHT-LTF 650.

The L-STF, L-LTF, L-SIG, and VHT-SIG sets 630 and 635 are transmittedthrough the 1st subchannel 610 and the 3rd subchannel 615, andthereafter the VHT-STF 640 and the VHT-LTF 650 of the STA 1 aretransmitted through the entire bonding channel. This process correspondsto a channel estimation process of the STA 1 for the full channelbandwidth. Thereafter, the VHT-STF 645 and the VHT-LTF 655 of the STA 2are transmitted through the entire bonding channel. Likewise, thisprocess corresponds to a channel estimation process of the STA 2 for thefull channel bandwidth.

As described above, the STA 1 and the STA 2 perform channel estimationat a time depending on a training offset, and exchange an RTS frame anda CTS frame with an AP through a subchannel. The STA 1 transmits the RTSframe 660 through the 1st subchannel 610, and the STA 2 transmits theRTS frame 665 through the 3rd subchannel 615. Thereafter, as shown inFIG. 5, the STA 1 and the STA 2 transmit a data frame according toinformation included in the CTS frame.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method for a wireless local area network, themethod comprising: transmitting, by a transmitter, a Very HighThroughput (VHT) signal field in a first bandwidth, the VHT signal fieldcomprising a bandwidth field and a training offset field, the bandwidthfield indicating a second bandwidth; and transmitting, by thetransmitter, at least one VHT-long training field (VHT-LTF) used forestimating a multiple input multiple output (MIMO) channel in the secondbandwidth indicated by the bandwidth field after transmitting the VHTsignal field, wherein the training offset field is set to: a first valuewhen the at least one VHT-LTF is transmitted for a single receiver; anda second value when the at least one VHT-LTF is transmitted for aplurality of receivers.
 2. The method of claim 1, further comprising:transmitting, by the transmitter, at least one VHT-short training field(VHT-STF) in the second bandwidth before transmitting the at least oneVHT-LTF.
 3. The method of claim 2, wherein the VHT signal field, the atleast one VHT-STF and the at least one VHT-LTF are transmitted as aphysical layer protocol data unit (PPDU).
 4. The method of claim 1,wherein the first bandwidth is equal to or smaller than the secondbandwidth.
 5. The method of claim 1, wherein the first bandwidth is 20MHz.
 6. The method of claim 1, wherein the second bandwidth is 20 MHz,40 MHz or 80 MHz.
 7. A device for a wireless local area network, thedevice comprising: a transceiver configured to transmit and receiveradio signals; and a processor operatively coupled to the transceiverand configured to: transmit, via the transceiver, a Very High Throughput(VHT) signal field in a first bandwidth, the VHT signal field comprisinga bandwidth field and a training offset field, the bandwidth fieldindicating a second bandwidth; and transmit, via the transceiver, atleast one VHT-long training field (VHT-LTF) used for estimating amultiple input multiple output (MIMO) channel in the second bandwidthindicated by the bandwidth field after transmitting the VHT signalfield, wherein the training offset field is set to: a first value whenthe at least one VHT-LTF is transmitted for a single receiver; and asecond value when the at least one VHT-LTF is transmitted for aplurality of receivers.
 8. The device of claim 7, wherein the processoris further configured to: transmit, via the transceiver, at least oneVHT-short training field (VHT-STF) in the second bandwidth beforetransmitting the at least one VHT-LTF.
 9. The device of claim 8, whereinthe VHT signal field, the at least one VHT-STF and the at least oneVHT-LTF are transmitted as a physical layer protocol data unit (PPDU).10. The device of claim 7, wherein the first bandwidth is equal to orsmaller than the second bandwidth.
 11. The device of claim 7, whereinthe first bandwidth is 20 MHz.
 12. A method for a wireless local areanetwork, the method comprising: receiving, by a receiver, a Very HighThroughput (VHT) signal field in a first bandwidth, the VHT signal fieldcomprising a bandwidth field and a training offset field, the bandwidthfield indicating a second bandwidth; receiving, by the receiver, atleast one VHT-long training field (VHT-LTF) used for estimating amultiple input multiple output (MIMO) channel in the second bandwidthindicated by the bandwidth field after transmitting the VHT signalfield; and estimating, by the receiver, a multiple input multiple output(MIMO) channel based on the at least one VHT-LTF, wherein the trainingoffset field is set to: a first value when the at least one VHT-LTF istransmitted by a transmitting device for a single receiver, the receivercorresponding to the single receiver; and a second value when the atleast one VHT-LTF is transmitted by the transmitting device for aplurality of receivers including the receiver.
 13. The method of claim12, further comprising: receiving, by the receiver, at least oneVHT-short training field (VHT-STF) in the second bandwidth beforereceiving the at least one VHT-LTF.
 14. The method of claim 13, whereinthe VHT signal field, the at least one VHT-STF and the at least oneVHT-LTF are received as a physical layer protocol data unit (PPDU). 15.The method of claim 12, wherein the first bandwidth is equal to orsmaller than the second bandwidth.
 16. The method of claim 12, whereinthe first bandwidth is 20 MHz.
 17. The method of claim 12, wherein thesecond bandwidth is 20 MHz, 40 MHz or 80 MHz.
 18. A device for awireless local area network, the device comprising: a transceiverconfigured to transmit and receive radio signals; and a processoroperatively coupled to the transceiver and configured to: receive, viathe transceiver, a Very High Throughput (VHT) signal field in a firstbandwidth, the VHT signal field comprising a bandwidth field and atraining offset field, the bandwidth field indicating a secondbandwidth; receive, via the transceiver, at least one VHT-long trainingfield (VHT-LTF) used for estimating a multiple input multiple output(MIMO) channel in the second bandwidth indicated by the bandwidth fieldafter transmitting the VHT signal field; and estimate a multiple inputmultiple output (MIMO) channel based on the at least one VHT-LTF,wherein the training offset field is set to: a first value when the atleast one VHT-LTF is transmitted by a transmitting device for a singlereceiver, the device corresponding to the single receiver; and a secondvalue when the at least one VHT-LTF is transmitted by the transmittingdevice for a plurality of receivers including the device.
 19. The deviceof claim 18, wherein the processor is further configured to: receiver,via the transceiver, at least one VHT-short training field (VHT-STF) inthe second bandwidth before receiving the at least one VHT-LTF.
 20. Thedevice of claim 19, wherein the VHT signal field, the at least oneVHT-STF and the at least one VHT-LTF are received as a physical layerprotocol data unit (PPDU).